Synthesis of cyclic oligomers from histidine-derived building blocks using dynamic combinatorial chemistry

Article abstract of DOI:10.1021/jo701832m

(Chemical Equation Presented) Histidine-derived hydrazide acetal monomers (3-dimethoxymethylbenzoyl)-L-histidine methyl ester 1 and (3- dimethoxymethylbenzoyl)-τ-benzyl-L-histidine methyl ester 2 were prepared from a histidine methyl ester and a τ-benzyl-

Full text of DOI:10.1021/jo701832m

Synthesis of Cyclic Oligomers from Histidine-Derived Building
Blocks Using Dynamic Combinatorial Chemistry
Masaomi Matsumoto and Kenneth M. Nicholas*
Department of Chemistry and Biochemistry, UniVersity of Oklahoma, 620 Parrington OVal,
Norman, Oklahoma 73019
knicholas@ou.edu
ReceiVed August 24, 2007
Histidine-derived hydrazide acetal monomers (3-dimethoxymethylbenzoyl)-L-histidine methyl ester 1 and
(3-dimethoxymethylbenzoyl)-τ-benzyl-L-histidine methyl ester 2 were prepared from a histidine methyl
ester and a τ-benzyl-histidine methyl ester by N-acylation with 3-(dimethoxymethyl)benzoic acid (3)
followed by hydrazinolysis. Acid-promoted hydrolysis of each acetal hydrazide initially produced a library
of cyclic oligomers that eventually converted to a cyclic dimer. The cyclic dimers 12 and 22 were
spectroscopically characterized and found to direct their imidazole-bearing sidechains outward (exo). No
evidence for templating the cyclic oligomers was observed using various metal ions and anionic substrates.
The average of pKa and pKa of dimer 12 was determined by potentiometric titration to be 6.6. Dimer 12
1
2
was found to catalyze the hydrolysis of p-nitrophenylacetate 10 times faster than 4-methyl imidazole.
Introduction
building blocks so that these artificial systems might reflect the
functional group diversity of the proteinogenic amino acids. Our
interest in a histidine-based building block 1 is due to the
importance of histidine in enzyme active sites as an acid/base
residue12 and as a ligand for transition metals in metallo-
enzymes.13-15 There are a number of examples of dynamic
combinatorial chemistry that exploit relatively strong transition
metal-ligand interactions to achieve templating.6,16-24 Our long-
Dynamic combinatorial chemistry, the synthesis of complex
molecules from simple building blocks via reversible linkages,
can give access to structures that would be difficult or impossible
to synthesize by traditional means. In a dynamic combinatorial
library (DCL), a species that is otherwise disfavored can be
amplified in the presence of a template with which it forms a
supramolecular complex.1-5 This change in equilibrium upon
addition of a template has been used by Lehn et al., Sanders et
al., Otto and Kubik, and others to produce many complex
molecules.6-9 Especially interesting is the generation of catalytic
species using transition-state analogues as templates.10,11
Toward the generation of DCLs that produce catalytic species,
it would be highly desirable to add to the selection of available
(7) Cousins, G. R.; Poulsen, S.; Sanders, J. K. M. Chem. Commun.
(Cambridge, U.K.) 1999, 1575-1576.
(8) Corbett, P. T.; Tong, L. H.; Sanders, J. K. M.; Otto, S. J. Am. Chem.
Soc. 2005, 127, 8902-8903.
(9) Otto, S.; Kubik, S. J. Am. Chem. Soc. 2003, 125, 7804-7805.
(10) Brisig, B.; Sanders, J. K. M.; Otto, S. Angew. Chem. 2003, 115,
1308-1311.
(11) Vial, L.; Sanders, J. K. M.; Otto, S. New J. Chem. 2005, 29, 1001-
1003.
* Corresponding author. Tel: (405) 325-3696; fax: (405) 325-6111.
(1) Ganesan, A. Angew. Chem., Int. Ed. 1998, 37, 2828-2831.
(2) Lehn, J. M. Chem.sEur. J. 1999, 5, 2455-2463.
(3) Lehn, J. M.; Eliseev, A. V. Science (Washington, DC, U.S.) 2001,
291, 2331-2332.
(4) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R.; Sanders, J. K. M.;
Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 898-952.
(5) Corbett, P. T.; Leclaire, J.; Vial, L.; West, K. R.; Wietor, J.-L.;
Sanders, J. K. M.; Otto, S. Chem. ReV. 2006, 106, 3652-3711.
(6) Giuseppone, N.; Schmitt, J. L.; Lehn, J. M. Angew. Chem., Int. Ed.
2004, 43, 4902-4906.
(12) Schneider, F. Angew. Chem., Int. Ed. Engl. 1978, 17, 583-592.
(13) Klinman, J. P. Chem. ReV. 1996, 96, 2541-2561.
(14) Costas, M.; Mehn, M.; Jensen, M.; Que, L. Chem. ReV. 2003, 104,
939-986.
(15) Abu-Omar, M. M.; Loaiza, A.; Hontzeas, N. Chem. ReV. 2005, 105,
2227-2252.
(16) Huc, I.; Krische, M. J.; Funeriu, D. P.; Lehn, J. M. Eur. J. Inorg.
Chem. 1999, 1415-1420.
(17) Giuseppone, N.; Lehn, J. M. J. Am. Chem. Soc. 2004, 126, 11448-
11449.
10.1021/jo701832m CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/30/2007
9308
J. Org. Chem. 2007, 72, 9308-9313

Cyclic Oligomers from Histidine-DeriVed Building Blocks
FIGURE 1. Synthesis of mHis (1).
FIGURE 2. Oligomerization of mHis (1).
term goal is to use histidine-based building blocks to produce
metalloenzyme active-site mimics through templating on transi-
tion-metal-TSA complexes. Acid-catalyzed hydrazone ex-
change has been demonstrated to be a versatile reaction for DCL,
and the suitability of bifunctional hydrazide/aldehyde building
blocks synthesized from amino acids is well-established.7,18,23-28
Our work incorporates the histidine residue into a DCL building
block, providing entry to dynamic libraries of oligo-histidine
macrocycles of possible use as imidazole-based catalysts. To
our knowledge, this is the first DCL to incorporate histidine or
any other imidazole-bearing building block.
Results and Discussion
The histidine-derived pseudo-peptide hydrazide building
block mHis 1 was synthesized by a modification of literature
methods starting from 3-(dimethoxymethyl)-benzoic acid 3
(Figure 1).26 The acid was coupled to L-histidine methyl ester
dihydrochloride using carbonyldiimidazole as a coupling re-
agent.29 The crude product amide ester 4 was subjected to room
temperature hydrazinolysis to yield the mHis monomer 1 that
was characterized spectroscopically.
Treatment of monomer 1 (2.5 mM in 3:1 acetonitrile/water,
room temperature, 8 h) with an excess of TFA or HCl initially
produced a mixture of cyclic oligomers, primarily dimer, trimer,
and tetramer, as indicated by HPLC and ESI-MS (Figures 2
and 3). Longer reaction times led to nearly complete conversion
of the oligomeric mixture to the cyclic dimer. When a large
FIGURE 3. HPLC trace of reaction of 1 with 2 equiv of HCl at 8 h
(top) and 14 days (bottom). (A) Cyclic dimer; (B) cyclic tetramer; and
(C) cyclic trimer. All peaks were identified by ESI-MS.
excess of acid was used, equilibrium was reached in 15 h; with
a slight excess of acid, equilibrium was attained in 14 days.
A time-dependent H NMR experiment in which mHis (1)
(18) Choudhary, S.; Morrow, J. R. Angew. Chem., Int. Ed. 2002, 41,
4096-4098.
(19) Epstein, D. M.; Choudhary, S.; Churchill, M. R.; Keil, K. M.;
Eliseev, A. V.; Morrow, J. R. Inorg. Chem. 2001, 40, 1591-1596.
(20) Cacciapaglia, R.; Di Stefano, S.; Mandolini, L. J. Am. Chem. Soc.
2005, 127, 13666-13671.
(21) Kieran, A. L.; Bond, A. D.; Belenguer, A. M.; Sanders, J. K. M.
Chem. Commun. (Cambridge, U.K.) 2003, 2674-2675.
(22) Stulz, E.; Ng, Y. F.; Scott, S. M.; Sanders, J. K. M. Chem. Commun.
(Cambridge, U.K.) 2002, 524-525.
(23) Goral, V.; Nelen, M. I.; Eliseev, A. V.; Lehn, J. M. Proc. Natl.
Acad. Sci. U.S.A. 2001, 98, 1347-1352.
(24) Roberts, S. L.; Furlan, R. L.; Otto, S.; Sanders, J. K. M. Org. Biomol.
Chem. 2003, 1, 1625-1633.
1
was treated with excess d-TFA in CD3CN/D2O confirmed that
the initially formed aldehyde was consumed rapidly, leading to
the formation of a mixture of oligomers, with dimers and trimers
predominating (Figure 4). The multiplet at 5.7 ppm (labeled B
in Figure 4) is assigned to the cyclic dimer R-CH based on NMR
characterization of the isolated compound (vide infra); it is
observed to steadily increase in magnitude over time. The
multiplet at 5.1 ppm (labeled A in Figure 4) is assigned to the
cyclic trimer R-CH. It is seen emerging initially before eventu-
ally disappearing and is the only discernible R-CH-type signal
other than peak B. The presence of a cyclic trimer at this stage
is also apparent in the ESI-MS. These oligomeric species were
slowly consumed over 15 h to generate almost exclusively cyclic
dimers.
(25) Cousins, G. R.; Furlan, R. L.; Ng, Y. F.; Redman, J. E.; Sanders, J.
K. M. Angew. Chem., Int. Ed. 2001, 40, 423-428.
(26) Furlan, R. L.; Ng, Y. F.; Cousins, G. R.; Redman, J. E.; Sanders, J.
K. M. Tetrahedron 2002, 58, 771-778.
(27) Bornaghi, L. F.; Wilkinson, B. L.; Kiefel, M. J.; Poulsen, S.
Tetrahedron Lett. 2004, 45, 9281-9284.
(28) Lam, R. T.; Belenguer, A.; Roberts, S. L.; Naumann, C.; Jarrosson,
T.; Otto, S.; Sanders, J. K. M. Science (Washington, DC, U.S.) 2005, 308,
667-669.
The crude cyclic dimer hydrochloride salt 12 was recovered
from an equilibrium mixture by removal of solvent in vacuo
and was analyzed by ESI-MS and NMR. NOEs observed
(29) Gelinsky, M.; Vogler, R.; Vahrenkamp, H. Inorg. Chem. 2002, 41,
2560-2564.
J. Org. Chem, Vol. 72, No. 24, 2007 9309

Matsumoto and Nicholas
1
FIGURE 4. Time-dependent H NMR spectra showing evolution of macrocycle library derived from 1 to the predominantly dimeric state. (A)
Trimer R-CH; (B) dimer R-CH; (C) dimer imidazole C4/5; (D) dimer aryl; (E) dimer aryl; (F) dimer imine CH; (G) dimer aryl; (H) dimer imidazole
C2; and (I) linear oligomer aldehyde CH.
(II) triflate, cobalt (II) chloride, and iron (III) chloride) and
anions (diphenylphosphate and phenylphosphonic acid). These
experiments were carried out in a variety of mixtures of
acetonitrile and water, using 1.1 equiv of TFA or 100 mM
ammonium chloride buffer (pH 3) and analyzed by HPLC. We
observed no templating behavior in any of these experiments
as the final distribution of oligomeric species (largely dimer)
was unperturbed. We believe that in the case of the metal
template candidates, interactions between imidazole and metal
are suppressed by the fact that under the acidic conditions
required for component exchange, the imidazoles are protonated.
It is also possible that the thermodynamically favored cyclic
FIGURE 5. Proposed structure of cyclic dimers 12 and 22 based on
NOESY data.
dimer forms metal ion complexes that are as (or more) stable
than the higher oligomers, so no detectable change in the
equilibrium occurs in the presence of metal ion. As noted
previously, we found that the cyclic dimer under neutral
conditions formed various complexes with zinc that were
detected by ESI-MS. In the case of anionic candidates, it is
likely that there is no appreciable advantage to the trimeric or
tetrameric cationic oligomer over the dimeric cationic oligomer
for anion binding, especially considering that all species are
strongly solvated in the largely aqueous experimental medium.
To explore the effect of substitution on the histidine imida-
zole, and to allow for library generation and templating studies
in nonaqueous solvent systems, the N-benzyl-histidine derivative
FIGURE 6. PM3 calculated structure of cyclic dimer 12.
of the monomer was synthesized according to Figure 7.
τ-Benzyl-L-histidine 5 was converted to the methyl ester 6 by
between the ortho-aryl C-Hs and the hydrazone C-H and the
R-amino acid C-H suggest that the structure of the dimer is as
illustrated in Figure 5. Molecular mechanics and gas-phase PM3
models30 of the cyclic dimer dication (Figure 6) are in agreement
with this proposed structure with the two imidazolium units
directed exo. The all-dimer state is most likely favored entropi-
cally at these low concentrations. The cyclic dimer also was
observed by ESI-MS to form various complexes, with Zn(II)
in methanolic solution, specifically, [(12)2Zn]2+, [(12)3(-H)
refluxing in methanolic hydrogen chloride. The resulting crude
ester dihydrochloride was freed with TEA and coupled to
3-(dimethoxymethyl)benzoic acid 3 using EDC and HOBT in
THF to give ester 7. The hydrazide monomer 2 was prepared
by hydrazinolysis of the ester 7 overnight in methanol at room
temperature. The resulting monomer mBnHis 2 was found to
be soluble in polar organic solvents such as chloroform.
The benzyl-substituted monomer 2 was found to react to form
cyclic oligomers in a similar fashion to the parent monomer.
When treated at 5 mM concentration with excess TFA in 75%
acetonitrile in water, the monomer produced a library consisting
of cyclic dimer, cyclic trimer, and cyclic tetramer, which were
detected by LC/ESI-MS. The evolution of the mixture was also
followed by proton NMR (Figure 8). Over a 60 h period, the
oligomers were consumed to produce only the cyclic dimer.
The BnHis dimer 22 could be produced and isolated on a
preparative scale by treating a 3:1 acetonitrile/water solution
with excess trifluoroacetic acid at room temperature for 10 days.
Zn2]3+, and [(12)2(-2H)Zn2]2+
.
In an attempt to perturb the equilibrium in the mHis library
away from the all-dimer state, experiments were carried out in
which template candidates were added to the oligomer libraries
or to the starting mixture of monomer and acid under various
conditions. Template candidates that we explored include metal
ions (zinc triflate, cadmium chloride, mercuric chloride, copper
(30) Wavefunction, Inc.: Irvine, CA, 2005.
9310 J. Org. Chem., Vol. 72, No. 24, 2007

Cyclic Oligomers from Histidine-DeriVed Building Blocks
FIGURE 7. Synthesis of 2.
FIGURE 8. Time-dependent 1H NMR of oligomer library derived from 2. (A) Trimer histidine R proton; (B) benzylic protons; (C) dimer histidine
R protons; (D) dimer imine CH; (E) dimer aromatic CH; and (F) dimer imidazole 5 position.
Solvent evaporation left the dimer in moderately high purity as
a ditrifluoroacetate salt. NOESY data for the BnHis cyclic dimer
22 (Figure 5) reveal an NOE enhancement between the histidine
R proton (5.8 ppm) and the 2 position of the 3-iminyl-benzamide
moiety (8.7 ppm), indicating the proximity of the histidine R
proton to the aromatic ring 2 position. This suggests that the
cyclic oligomer is arranged in such a way as to place the
histidine side chain away from the aromatic backbone and the
R proton toward the aromatic backbone. In addition, we observe
an NOE interaction between the imine CH and the aromatic
ring’s 4 position, suggesting the aromatic imine conformation
illustrated in Figure 5. The structure of the cyclic dimer derived
from 2 is therefore analogous to the structure of the non-
benzylated mHis cyclic dimer.
Templating experiments using the τ-benzyl-L-histidine-
derived monomer 2 revealed no templating behavior. These
experiments were carried out by adding template candidates to
a pre-equilibrated library of cyclic dimer 22 or combining
monomer 2 and template candidates in various solvents and
treating with 1.1 equiv of TFA. Samples were monitored by
HPLC. Template candidates tested include metal ions and
anionic templates analogously with previous experiments using
monomer 1. All mixtures yielded the cyclic dimer at equilibrium.
The oligomers are protonated under these experimental condi-
tions, and it is likely that no interaction with metal takes place.
In the case of anionic templates, it is likely that the ion-ion
interactions we were attempting to exploit are not sufficiently
discriminating to select higher oligomers over dimers.
pKa of mHis Cyclic Dimer. We wished to determine if the
presence of two imidazole moieties on the macrocycle would
lead to any perturbation in basicity. To characterize the acid-
base properties of the mHis cyclic dimer, a solution of the dimer
in dilute aqueous HCl was titrated with sodium hydroxide
solution, and the pH was monitored by a potentiometric probe
as a function of added base. Over the addition of 2 equiv of
sodium hydroxide, there was only one discrete endpoint visible.
We interpret this to mean that the pKa values for first and second
protonation are quite close together. The average of the pKa of
the first and second protonation of the cyclic dimer appears to
be approximately 6.6 (see Figure 1 in the Supporting Informa-
tion), as compared to 7.45 for 4-methyl imidazole.31 This is
most likely due to a depression of the pKa of the second
protonation, possibly due to electrostatic interactions between
the two imidazole moieties.
(31) Bruice, T. C.; Schmir, G. L. J. Am. Chem. Soc. 1957, 80, 148.
J. Org. Chem, Vol. 72, No. 24, 2007 9311

Matsumoto and Nicholas
interactions. In nature, histidine plays many catalytic roles that
take advantage of the chemical versatility of imidazole. Mono-
mers of this type may prove useful in establishing DCLs with
a biomimetic functional diversity.
FIGURE 9. Catalytic hydrolysis of p-nitrophenylacetate.
Experimental Section
TABLE 1. Hydrolysis of p-Nitrophenyl Acetate Catalyzed by
Dimer 12 and 4-Methyl Imidazole
3-(Dimethoxymethyl)-N-benzyl-histidine Methyl Ester (4).
Carbonyl diimidazole (1.82 g, 11.3 mmol) was dissolved in 45 mL
of dry ethyl acetate. 3-Carboxybenzaldehyde dimethoxyacetal (3)
(2.19 g, 11.2 mmol) was added to this solution, and the mixture
was stirred under nitrogen for 3 h. The solution was refluxed
overnight and then cooled. L-Histidine methyl ester dihydrochloride
(4.06 g, 16.8 mmol) was added, followed by 6 mL (43 mmol) of
triethylamine. The mixture was stirred for 2 weeks at room
temperature. The mixture was then diluted with 50 mL of
chloroform and washed with saturated aqueous sodium bicarbonate
(two portions of 50 mL). The resulting organic phase was
evaporated in vacuo to produce 3.13 g of 3-(dimethoxymethyl)-
benzoyl-histidine methyl ester as a light yellow foaming oil (80%
crude yield). This material was sufficiently pure to carry to the
12 cyclic dimer,
k2 (M-1 s-1)
4-methyl imidazole,
background,
pH
k2 (M-1 s-1
)
k1 (s-1)
6.2
6.6
7.0
0.26
0.83
0.88
0.022
0.083
0.095
8.4 × 10-6
1.7 × 10-5
1.7 × 10-5
Hydrolysis of p-Nitrophenyl Acetate Catalyzed by mHis
Cyclic Dimer. Since the mHis cyclic dimer possesses two
imidazole moieties, we wondered whether it would display
hydrolytic activity toward p-nitrophenylacetate. Catalysis of
p-nitrophenylacetate hydrolysis by the mHis cyclic dimer was
investigated by spectrophotometric assay at 320 nm. Reactions
were carried out in 100 mM bis-Tris buffer at pH 6.20, 6.60,
and 7.00 at 25 °C. The k2 values for the mHis cyclic dimer at
pH 6.2, 6.6, and 7.0 were found to be 0.22, 0.83, and 0.95 s-1
M-1, respectively. These values are approximately 5 times the
appropriate k2 values for the electronically similar 4-methyl
imidazole, when adjusted for imidazole equivalents (Figure 9
and Table 1). Since the two imidazole units of the dimers are
oriented exo, there would be no opportunity for cooperative acid/
base or acid/nucleophile catalysis. The relatively modest
observed catalytic rate enhancement for the cyclic dimer relative
to the simple imidazole could be explained by a greater effective
concentration of a free nucleophilic imidazole unit in the dimer
at pH 6.2-7.0 due to its greater acidity. Although our results
are not directly comparable to existing peptidic artificial
esterases incorporating histidines32,33 due to differences in
experimental conditions, qualitatively, the rate enhancements
we observed are relatively small.
1
next step. H NMR: 300 MHz in CDCl3; 7.97 (1H, s), 7.85 (1H,
d, J ) 6.3 Hz), 7.62-7.64 (3H, m), 7.45 (1H, t, J ) 7.5 Hz), 6.87
(1H, s), 5.43 (1H, s), 5.00 (dt, J ) 7.5, J ) 5.1), 3.73 (3H, s), 3.33
(6H), 3.24 (2H, pseudo-t, J ) 9.3); HRMS (ESI): 348.1533, Calcd
mass (C17H22N3O5) 348.1559.
3-(Dimethoxymethyl)-N-benzyl-histidine Hydrazide (1) (mHis).
Crude 3-(dimethoxymethyl)benzoyl-histidine methyl ester (4) (3.13
g, 9.00 mmol) was dissolved in 80 mL of methanol, filtered through
filter paper, and treated with 2.5 mL of hydrazine monohydrate.
After 0.5 h stirring at room temperature, a white precipitate was
observed. The mixture was stirred for 72 h, evaporated in vacuo,
and triturated 3 times with methanol. The insoluble solid product
1 was collected (2.64 g, 84% yield, 67% over two steps). 1H
NMR: 300 MHz in 50% v/v CD3CN/D2O: 7.76 (1H, s), 7.73 (1H,
d, J ) 9.3) 7.54-7.55 (2H, m), 7.45 (1H, t, J ) 7.5), 6.87 (1H, s),
5.38 (1H s), 4.70 (1H, dd, J ) 8.7, J ) 6.0), 3.28 (6H), 3.03 (2H,
m); 13C NMR: 75.45 MHz in 50% v/v CD3CN/D2O: 172.5, 168.95,
139.8, 136.4, 134.7, 134.5, 131.1, 129.7, 128.6, 126.6, 117.5, 104.0,
54.0, 53.9, 30.1; IR: νmax/cm-1 3280, 3042, 2951, 2834, 1660, 1638,
1547; HRMS (ESI): 348.1663, Calcd mass (C16H22N5O4) 348.1672.
Synthesis of τ-Benzyl-L-histidine Methyl Ester Dihydrochlo-
ride (6). τ-Benzyl-L-histidine (5) (6.50 g, 26.5 mmol) was placed
in 100 mL of 3 M methanolic HCl and refluxed for 2 h. Solvent
was removed under vacuum giving the crude τ-benzyl-L-histidine
methyl ester dihydrochloride (6) (8.57 g, 97%). ESI-MS: 260.1397,
M + H+ (theoretical, 260.1399). 300 MHz 1H NMR in D2O: 8.67
(1H, d, J ) 1.8 Hz), 7.26-7.36 (6H, m), 5.26 (2H, s), 4.30 (1H, t,
J ) 6.9 Hz), 3.60, (3H, s), 3.26, (2H, d, J ) 7.2 Hz). 13C NMR in
D2O: 169.5, 139.5, 136.1, 130.3, 129.5, 134.5, 128.4, 121.8, 54.7,
53.7, 52.6, 25.9.
Conclusion
We have synthesized two histidine-derived pseudo-peptide
monomers that undergo thermodynamically controlled oligo-
merization under acidic conditions. Although the initial distribu-
tion of oligomers for both non-benzylated (1n) and benzylated
(2n) systems included cyclic dimers, trimers, and tetramers, at
equilibrium both systems led exclusively to the cyclic dimer.
The imidazole units of the cyclic dimers 12 and 22 are oriented
exo based on solution NMR studies. These results demonstrate
that the monomer mHis can be incorporated into cyclic
oligomers by dynamic combinatorial chemistry. The average
pKa value of cyclic dimer 12 (6.6) is significantly lower that
that of 4-methyl imidazole (7.45). Dimer 12 also catalyzes the
hydrolysis of p-nitrophenyl acetate 5 times faster (on a normal-
ized basis) than 4-methyl imidazole. No conditions were found
in which the library could be diverted from the all-dimer state
by templating. Screening of appropriate template compounds
is ongoing, as well as exploration of alternative exchange
reactions that might be more compatible with metal-imidazole
Synthesis of 3-(Dimethoxymethyl)benzoyl-τ-benzyl-L-histidine
Methyl Ester (7). τ-Benzyl-L-histidine methyl ester dihydrochloride
(6) (0.85 g, 3.0 mmol), 3-(dimethoxymethyl)benzoic acid (3), (0.50
g, 3.0 mmol), and 1-hydroxybenzotriazole hydrate (0.40 g, 3.0
mmol) were placed in 100 mL of dry THF and cooled in an ice
bath. Triethylamine (1.18 mL, 9.9 mmol) was added, followed by
N-(3-diethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (0.49
g, 3.0 mmol). The reaction mixture was stirred for 4 days. Solvent
was removed under vacuum. The crude residue was reconstituted
in 200 mL of chloroform and washed with three 10 mL portions
of saturated sodium bicarbonate. The organic portion was dried
with magnesium sulfate and evaporated to brown crude oil. This
was chromatographed on silica eluted with ethyl acetate to give
0.81 g of yellow oil (61.5%). ESI-MS m/z 438.1937 (theoretical
(32) Broo, K. S.; Brive, L.; Ahlberg, P.; Baltzer, L. J. Am. Chem. Soc.
1997, 119, 11362-11372.
1
M + H+ 438.2029). 300 MHz H NMR in CDCl3: 8.31 (1H, d J
(33) Nicoll, A. J.; Allemann, R. K. Org. Biomol. Chem. 2004, 2, 2175-
) 7.5 Hz), 7.98 (1H, s), 7.83 (1H, d, J ) 8.1 Hz), 7.59 (1H, d, J
) 7.8 Hz), 7.48 (1H, s), 7.42 (1H, t, J ) 8.1 Hz), 7.24-7.33 (3H,
2180.
(34) Jain, R.; Cohen, L. A. Tetrahedron 1996, 52, 5363-5370.
9312 J. Org. Chem., Vol. 72, No. 24, 2007

Cyclic Oligomers from Histidine-DeriVed Building Blocks
m), 7.08-7.11 (2H, m), 6.68 (1H, s), 5.41 (1H, s), 5.04 (2H, s),
4.95 (1H, m), 3.64 (3H, s), 3.31 (6H, s), 3.14 (2H, m). 13C NMR:
172.2, 167.1, 138.8, 138.3, 137.4, 136.1, 134.3, 130.0, 129.3, 128.7,
128.6, 127.6, 127.5, 126.2, 117.3, 102.8, 53.2, 53.0, 52.9, 52.5,
51.1, 29.7.
7.41, (2H, m), 7.34, (1H, t, J ) 7.2 Hz), 7.28-7.15, (5H, m), 5.83,
(1H, m), 5.35, (2H, m), 3.36-3.29, (1H, m), 3.22-3.14, (1H, m).
13C NMR: 172.5, 168.8, 145.0, 136.4, 136.3, 136.0, 135.6, 134.0,
133.1, 130.3, 130.1, 129.91, 129.2, 128.5, 126.1, 121.8, 53.8, 50.8,
27.9. NOESY: strong NOE cross-peaks are present between R
proton (5.83) and aromatic 2 position (8.66) and also between imine
CH (7.84) and aromatic 4 position (7.41), indicating a conformation
in which the imidazole-bearing sidechains are oriented outward from
the macrocyclic structure. ESI-MS: m/z 747.3380; M + H+ (calcd
747.3156).
Templating Experiments for 1 and 2. Solutions of monomer
(1 or 2, 5 mM) were prepared in 75% acetonitrile/25% water and
treated with a stock solution of TFA in acetonitrile to bring the
amount of TFA to 1.5 equiv in relation to the monomer. Templating
agents were added to these solutions using stock solutions in 0.33,
1, and 10 equiv quantities. Templating agents used included zinc
triflate, copper (II) triflate, cobalt (II) chloride, nickel (II) chloride,
iron (II) chloride, cadmium (II) chloride, and mercury (II) chloride.
Organic templates that were used included diphenylphosphate,
trimesic acid, phosphoryl choline, p-toluenesulfonic acid, and
phosphoric acid. Samples were analyzed over days by diluting in
water (10:1) followed by HPLC analysis.
Catalysis of p-Nitrophenylacetate Hydrolysis by mHis Cyclic
Dimer. All reactions were carried out in 100 mM bis-Tris buffer
and analyzed spectrophotometrically at 320 nm at 25 °C in a 1 cm
cuvette. Assays were carried out at pH 6.20, 6.60, and 7.00. The
p-nitrophenylacetate concentration was 2.69 × 10-4 M. 4-Methyl
imidazole and mHis cyclic dimer concentrations were varied. First-
order constants k1 were calculated from initial rates and substrate
concentrations. First-order constants k1 were plotted against the
catalyst concentration to give the apparent second-order constant
k2.
Synthesis of 3-(Dimethoxymethyl)benzoyl-τ-benzyl-L-histidine
Hydrazide (2). 3-(Dimethoxymethyl)benzoyl-τ-benzyl-L-histidine
methyl ester (7) (0.81 g, 1.85 mmol) was dissolved in 20 mL of
dry methanol and treated with 1 mL of hydrazine monohydrate.
After stirring for 3 days at room temperature, solvent was removed
under vacuum, and the crude product was chromatographed on silica
(9:1 dichloromethane/methanol) to give 0.405 g (50%) of 2. ESI-
1
MS m/z 438.2112 (theoretical M + H+ 438.2143). 300 MHz H
NMR in CDCl3: 8.54, (1H, d, J ) 6.6 Hz), 7.96 (1H, s), 7.82 (1H,
d, J ) 7.8 Hz), 7.57 (1H, d, J ) 7.5 Hz), 7.37-7.45 (2H, m),
7.24-7.30 (3H, m), 7.07-7.10 (2H, m), 6.74 (1H, s), 5.39
(1H, s), 5.00 (2H, s), 4.84 (1H, q, J ) 5.4 Hz), 3.29 (6H, s), 2.90-
3.18 (2H, m). 13C NMR: 171.8, 167.2, 138.7, 138.4, 136.9, 135.9,
133.7, 130.1, 129.1, 128.7, 18.4, 127.5, 127.3, 125.8, 117.5, 102.5,
53.0, 52.8, 52.7, 51.1, 29.6.
Oligomerization of 1 (mHis Monomer). Analytical scale:
monomer 1 (1.0 mg, 2.9 µmol) was placed in 1 mL of 75%
acetonitrile v/v aqueous solution (2.88 mM). This was treated with
10 µL of TFA (0.13 mmol, 46-fold excess) or 60 µL of 53.7 mM
stock solution (0.2 mL in 50 mL) of TFA in water (1.1 equiv of
TFA with respect to 1). Preparative scale: 1 (100 mg, 288 µmol)
was placed in 100 mL of 75% acetonitrile v/v aqueous solution.
Concentrated hydrochloric acid (0.05 M) was added, and the
solution was stirred for 2 weeks. The mixture was evaporated in
vacuo to produce a white amorphous solid (crude cyclic dimer 12
hydrochloride salt). No further purification was required. 1H NMR
300 MHz in CD3OD: δH 8.75 (1H, s), 8.69 (1H, s), 7.81 (1H, s),
7.69 (1H, pseudo-dt, J ) 6.9, 1.8 Hz), 7.35-7.43 (3H, m), 5.76
(1H pseudo-t, J ) 7.2 Hz), 3.40 (1H, dd, J ) 15.6, J ) 6.3 Hz),
3.21 (1H, dd, J ) 15.3, 7.5 Hz); 13C NMR:100.57 MHz in CD3-
OD: 171.0, 167.1, 143.3, 134.7, 133.1, 132.5, 131.0, 130.6, 128.3,
127.1, 124.2, 117.3, 49.7, 26.0. ESI-MS: m/z 284.1, 284.6; HRMS
(ESI): 567.2278, Calcd mass (C28H27N10O4) 567.2211.
Acknowledgment. We gratefully acknowledge the U.S.
Department of Education for financial support through the
GAANN Fellowship and Prof. J. K. M. Sanders for helpful
discussions. We also thank Dr. Ana Belenguer for valuable
advice on HPLC techniques.
PreparativeSynthesisofBnHisCyclicDimer22.3-(Dimethoxym-
ethyl)benzoyl-τ-benzyl-L-histidine hydrazide 2 (100 mg, 0.23 mmol)
was dissolved in 40 mL of 75% acetonitrile/25% water v/v. This
solution was treated with 0.5 mL of TFA. After 10 days, solvent
was removed under high vacuum, and the residue was dried under
sodium hydroxide to give 89 mg (0.091 mmol, 79%) of the τ-BnHis
cyclic dimer diTFA acid salt.
Supporting Information Available: Spectral characterization
1
of all new compounds, including H, 13C, and 2-D NMR spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
300 MHz H NMR in d4-methanol: 8.93 (1H, d, J ) 1.5 Hz),
8.66 (1H, s), 7.84 (1H, s), 7.56 (1H, dt, J ) 8.1 Hz, 0.9 Hz), 7.44-
JO701832M
J. Org. Chem, Vol. 72, No. 24, 2007 9313